Massive multiple input and multiple output (Massive MIMO) systems have been widely accepted as one of the key gaming changing technologies for the next generation wireless communications, or 5G (5th Generation) wireless communication systems. The basic concept of Massive MIMO is to provide wireless communication services by utilizing a large number of radiating antennas with corresponding transmitters to service multiple users. By using many transmitters at the same site (centralized) or at different sites (distributed) simultaneously, the frequency spectrum efficiency will be improved, so that the network capacity and data throughput can be significantly increased, thereby providing better services for more users.
Massive MIMO systems involve multiple data streams being multiplexed and mapped to different antennas, linear RF transceivers are required to provide the spatial multiplexing performance using computation-effective linear precoding algorithms. In a practical wireless system with physical RF transmitters, RF power amplifiers (PAs) are required to provide reasonable radiating power from each transmitter. Such amplifiers have a region where they operate linearly and a region beyond this where they do not. Operating in the non-linear region results in signals outside of the required bandwidth and in distortions in the signal. This results not only in the signal itself being difficult to decode, but also provides interference for neighbouring signals. Operating in the linear region addresses this, but operation in this region is not efficient. Thus, these amplifiers if used in a region where they are efficient introduce nonlinear distortion into the signal causing both in-band signal quality degradation (which causes problems to the transmitter itself) and out-of-band spectrum regrowth (which cause problems to others transmitters working in the adjacent frequency band).
To address this problem two approaches are commonly used:
1) Backing-off approach: in this approach, radio frequency power amplifiers are operated in the linear operating region, i.e., backing-off the power from the saturation operating region. A drawback of this conventional approach is that it leads to very low power efficiency.
2) Digital predistortion (DPD) approach: This involves the input signal being pre-distorted to compensate for distortions that will arise at the amplifier, such that the amplified signal has reduced distortions. FIG. 1A shows a conventional DPD-enabled RF transmitter with two signal paths, a forward data path and a feedback data path. The amplified radio frequency signal is sampled and fed back via sampling receiver devices. The attenuator is used for reducing the power of the feedback signal, a down-converter converts the signal from a high frequency radio signal to a lower frequency signal, and an analogue to digital converter returns the signal to a digital signal. This digital signal is then compared with the signal that generated it at the digital signal processing circuit and from this comparison the pre-distortion function applied to the data signal on the forward data path is updated if required to mitigate for any differences detected between the two signals. Due to its satisfactory linearization performance and flexibility, DPD has been largely used as a preferred option to reduce the nonlinear distortion introduced by the RF PAs when driving RF PAs into nonlinear saturation region.
However, directly applying this DPD architecture for multiple RF transmitters based system (like Massive MIMO system) results in a system that is expensive both in hardware and in power. FIG. 1B shows such a device.